1. Field of the Invention
The present invention relates to a method for manufacturing a rocker arm for opening and closing a valve in a valve operating system for use in an internal combustion engine for vehicles for example.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
FIG. 10 is a cross-sectional view showing a valve operating system for use in an internal combustion engine for vehicles. As shown in this figure, provided in this valve operating system is a rocker arm 1 for converting a rotary motion of a cam 3 which rotates in conjunction with a crankshaft 2 into a reciprocating motion of a valve stem 5 for a valve such as an intake valve or an exhaust valve.
As shown in FIGS. 10 and 11, this rocker arm 1 is provided with a pair of side walls 1a and 1a disposed in parallel with each other, connecting walls 1b and 1c connecting one end of the side walls and the other end thereof respectively, a valve stem contacting portion 1d which is attached to the one end side wall 1b and is to be brought into contact with the valve stem 5, and a pivot engaging portion 1e which is attached to the other end side wall 1c and is to be engaged with a pivot portion 6 fixed to a cylinder head. Both the side walls 1a and 1a of this rocker arm 1 are provided with shaft fixing apertures 1f and 1f on the same axis line into which a supporting shaft 8 of a roller 7 is fixedly inserted, so that the roller 7 and the cam 3 are brought into a rolling contact with each other.
This kind of rocker arm has been conventionally manufactured by forging or precision casting (lost-wax processing). However, in accordance with the recent year's movement toward high revolution and high power of an internal combustion engine, it is required to attain lightweighting of a rocker arm to reduce the inertia-weight, and therefore there are a number of proposals on press working techniques for manufacturing a rocker arm.
In a method for manufacturing a rocker arm by press working, as a method for forming a shaft fixing aperture 1f, cutting using a drill or press blanking using a punch is generally employed.
However, cutting work causes deteriorated production efficiency and an increased production cost due to the lower efficiency as compared with press working, and also may exert a harmful influence by chips generated during the cutting work.
On the other hand, in a press blanking method, although the production efficiency can be improved, sufficient flatness of the inner peripheral surface of the punched aperture cannot be secured, which makes it difficult to secure, e.g., sufficiently wide sheared surface in the inner peripheral surface. This causes a difficulty in assuredly bringing the inner peripheral surface into contact with the supporting shaft 8, which in turn may cause deteriorated shaft holding force.
Under the circumstances, the assignee of the present invention has developed a method for forming a shaft fixing aperture 1f as disclosed by Japanese Patent No. 3,497,368 (hereinafter referred to as “Patent Document 1”) and Japanese Patent No. 3,582,977(hereinafter referred to as “Patent Document 2”). In the method, a prepared aperture is initially formed by blanking, and then a shaving punch is driven into the prepared aperture to thereby obtain a smoothed inner peripheral surface of the aperture.
With this method, high productivity can be attained since the shaft fixing aperture 1f is formed only by press working using an aperture forming punch and a shaving punch, and a smoothed inner peripheral surface of the aperture by shaving can be obtained, resulting in sufficient shaft holding ability.
In the method for manufacturing a rocker arm disclosed by the aforementioned Patent Documents 1 and 2, a shaving punch used as a tool having a sharp cutting edge is used. However, a large load is intensively applied to the cutting edge during the processing, which causes an early abrasion of the cutting edge. Using such punch with an abraded cutting edge deteriorates the cutting workability, causing generation of burrs, which in turn causes deteriorated quality. As will be understood from the above, a shaving punch is short in life, and therefore it is required to replace with a new one in a short period of time. Thus, the method should be improved in terms of running costs.
Furthermore, in the method as disclosed in Patent Documents 1 and 2, since an inner peripheral portion of the prepared aperture is shaved off by shaving, chips will be generated, which in turn cause defects such as littering of chips and/or damages due to the chips. Accordingly, it is also required to take sufficient measures to remove such chips.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.